12V RV installation

aspn · September 2017

Hello, i just bought two solar panels for my rv.
The panels are in a different size and have different voltage.
thats why i need 2 solar controllers.

I am using 120W on Voltronic MPP 165 DUO Digital
and 60 W on Voltronic MPP 165 DUO Digital

I just have a question about the installation from the Controller to the Battery.

At the moment i have this installation

Image: https://us.v-cdn.net/6024911/uploads/editor/zf/er0z34ongzqn.png

from both Controllers is going a plus and minus cable to the battery.
But is it also possible to connect it like this?

Image: https://us.v-cdn.net/6024911/uploads/editor/28/ipedsj16i01g.png

That would be a "nicer" installation and less cables in the rv.
Sorry for the beginner question.

Hope someone can help me.

Thanks
aspn

littleharbor2 · September 2017

As long as the Vmp. of the panels is close you can parallel wire them and use one controller.

Estragon · September 2017

The short answer is yes.

The longer answer has a "but". The first issue is wire size. You need to make sure the wire carrying the combined current from the two controllers is heavy enough to carry the combined current. You would effectively be making the controller on the right battery terminals into a battery buss bar.

You should also have a properly sized fuse/breaker protecting the circuits, one for the heavier wire from buss to battery, and a smaller one between controllers.

If there is much voltage drop between the left and right controllers, there could be an issue where the right hand controller "sees" a different battery state of charge than the left. In that case, the left controller may want to supply absorb voltage too long. Probably not a big deal, but something to watch for.

As LH2 said, if the panels were close in Vmp, a single controller would suffice. You said they have different voltages though. I'm a bit surprised a 60w and 120w would be anything other than ~18Vmp. If they were, I'd use a single PWM controller, and call it a day.

aspn · September 2017

Good morning, thanks for your answers.

The 110W has 18,56V (Vmp) 21,9V (Voc)
and the 60 W has 20,88(Vmp) 24,64V(Voc)

I think the bigger problem is that the panels are placed on different spots on the van. That one is getting less light in the late hours than the other and then its more of a problem i think?

mcgivor · September 2017

Having both panels in parallel, using one controller should be fine, if one has less sun it simply would contribute less with little or no effect on the other, much lIke having a shaded panel in a parrallel array.

BB. · September 2017

Two things. Pick a clear noon day--Set up the panels (same angle, facing sun) and run three tests. Panel A output, Panel B output, Panel A+B output. See what the difference is (small difference, use one controller). Note, the battery bank has to be fairly discharged/have a large constant load. If the battery is >~80% state of charge and the controller is at Absorb (~14.75 volts or so), then the battery will be limiting the charging current, not the panels+configuration questions.

Second, if you have two (or more) charge controllers), I highly suggest that you do the first wiring. Each controller has its own "home run" to the battery bank. When you connect them in series (like #2), the added current and voltage drop can reduce charging current. And, some charge controllers do behave a bit squirrelly if they are connected to a "noisy" output--The noise from one controller can confuse the other controller and reduce the charging current.

And lastly, you should have fuses/breakers for all + wires that leave the battery bank (especially if you have a larger battery bank and/or smaller diameter wires). You want the fuse to blow vs the wiring get hot and possibly start a fire if there is a short circuit somewhere.

-Bill

aspn · September 2017

Great forum!!!
Thank you all for your tips.
@BB I will try that tomorrow.

Cheers,
Pat

Estragon · September 2017

@BB.

I have 2 charge controllers (2 classic 150s), soon to be 3 (adding a Morningstar PWM with vertically mounted panels to float over winter), all connected or to be connected to battery +/- buss bars. Also a couple of inverters. It seems to work okay.

The (parallel?) connection on the controller on the right in the second diagram seem to me as being not so different than a battery buss bar. When adding the PWM, should I be going directly to the battery, or add to the busses?

BB. · September 2017

It does depend on the wiring back to the battery bank. Longer run of smaller diameter wire is worse.

And battery bus bar/common connections have lower resistance to the battery bank (which also acts like a large filter capacitor which helps reduce controller cross-talk).

-Bill

Estragon · September 2017

IIRC the wire to the bank from buss is 4/0 and ~6', which, assuming good connections, should be pretty low resistance. So if I'm understanding correctly, the bank should still act as a capacitor even though the controllers are connected to a common buss?

BB. · September 2017

There is also the inductance of the wiring. A capacitor at one end, the "long" inductive wiring, and low resistance makes for a "High Q" oscillator. Hit that with square waves (PWM from charge controller) and you can have a nice oscillator.

Not saying that will be a problem here. Just an example of what can happen. Bundle tying the plus and minus called together lowers the cable inductance (which helps move the natural frequency higher. ... and out of resonates ).

-Bill

Estragon · September 2017

Ok, thanks. Will have to do some reading before finalizing the wiring on the new PWM and panels. Bundling the +/- wires to the bank would mean putting the batteries in a U shape instead of the straight line with +/- at each end, which the box may not accomodate.

BB. · September 2017

You do the best you can with what you have.

-Bill

12V RV installation

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